1. Field of the Invention
The present invention relates to an image sensing apparatus and a correction method, and more specifically to a technique for correcting the output of an image sensing device used in an image sensing apparatus.
2. Description of the Related Art
Conventionally, solid state image sensing devices such as CCD and CMOS sensors are generally used in digital cameras, video cameras, and other image sensing apparatuses. With these solid state image sensing devices, it is known that defective pixels occurring in the manufacturing process are one cause of a decreased image quality and decreased manufacturing yield. Given the difficulty in completely eliminating defective pixels, it is generally known that improvement in image quality can be achieved by interpolation using pixels around the defective pixel.
A known method for correcting signals output from defective pixels is disclosed in the Background Art of JP 2003-333435A, for example. Firstly, defective pixels are assessed, for example, when the solid state image sensing device is shipped from the factory, using output values obtained by exposing the solid state image sensing device for a standard charge accumulation period under prescribed conditions. Information acquired at this time showing the position and output levels of defective pixels is stored, and when image sensing is performed, the output of the defective pixels is interpolated using the output level of pixels adjacent to the defective pixels based on the stored position information and output level information.
On the other hand, dark current is known to occur with solid state image sensing devices due to factors such as temperature. The dark current changes depending on use environment and exposure period. For this reason, a known technique involves performing an image sensing operation with the image sensing device shielded from light immediately before or after the actual image sensing operation, and subtracting the sensed image obtained at this time (dark or black image) from the image taken of the subject (e.g., see Background Art of JP 2003-333435A). Subtracting a dark image from the subject image in this way reduces the effects caused by the fixed pattern noise of the dark current component and minute defective pixels, and allows improvement in image quality to be achieved.
A technique is also disclosed that involves determining noise conditions by comparing the output value of a dark image or the result of subtracting a dark image with a prescribed threshold value for each pixel, and performing correction after switching between subtraction and replacement of the dark image according to those conditions (e.g., see JP 2004-5424A). This technique makes it possible to reduce noise, by performing replacement, of even pixels approaching a saturation level at which correction by subtraction is difficult, and, moreover, to realize correction that redresses the problem of noise being newly superimposed as a result of the subtraction.
Further, a method is also disclosed in which a gain is applied to fixed pattern noise extracted from dark images and stored when a solid state image sensing device is manufactured so that the fixed pattern noise is made responsive to changes in temperature detected by a temperature sensor in the image sensing apparatus (e.g., see JP 2003-18475A). This method allows the effects caused by temperature-dependent fixed pattern noise to be removed by adding or subtracting fixed pattern noise to which a gain that depends on temperature change has been applied to correct an image taken of a subject.
A technique is also disclosed that involves correcting black levels in which the random component of dark images has been reduced, using data obtained by capturing and averaging dark image output data multiple times (e.g., see JP H8-65506A).
With the correction by subtracting dark images disclosed in JP 2003-333435A, effects caused by fixed pattern noise and two-dimensional dark current unevenness on the screen can be reduced. However, because this is merely subtraction involving sensed images and black images, noise due to the random component of both images is added, which may conversely prevent improvements in image quality.
In the case of the replacement of defective pixels as disclosed in JP 2004-5424A, the effects caused by the dark current component, which changes depending on temperature and exposure period, cannot be reduced. With the method disclosed in JP 2004-5424A, pixels that exceed a prescribed threshold value are replaced, having been judged to have excess noise. However, in the case where this threshold value is set based on the result of subtracting dark images from sensed images, noise cannot be accurately judged if luminance varies greatly in the sensed image. When sensing a starry sky or fireworks, for example, high brightness stars and fireworks may possibly disappear due to replacement processing.
Also, in the case where the dark current component has increased in a portion that includes a plurality of pixels due to the effects of peripheral circuitry or heat emitted from the image sensing device itself, appropriate processing cannot be carried out even if replacement is performed based on a threshold judgment. This is because with areas in which dark current has partially increased there will also be an increase in dark current around the periphery of the pixels to be replaced.
The method disclosed in JP 2003-18475 seeks to remove the effects caused by temperature change in the image sensing apparatus. However, since it is not the temperature of the actual image sensing device that is detected, and the temperature distribution of the image sensing device is not always uniform, correction cannot be performed with high precision.
Also, with the method disclosed in JP H8-65506, an excess amount of time is required to capture images multiple times. User operability is thus impaired due to the release interval being affected in image sensing apparatuses such as cameras.